Respiration — Edexcel International GCSE Science (Double Award)
How respiration releases energy from glucose to make ATP, aerobic and anaerobic respiration, and the seed practical.
Revision notes
4 short notes, in the order of the specification. Each one in short:
Respiration is the chemical process in every living cell that releases energy from food molecules such as glucose. The energy is used to join a phosphate group onto ADP to make ATP. ATP is then broken down to release energy for muscle contraction, active transport, making large molecules, cell division and nerve impulses. Respiration is not breathing.
Aerobic respiration uses oxygen to break down glucose and releases a large amount of energy: glucose + oxygen → carbon dioxide + water, or C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O. Anaerobic respiration happens without oxygen. In animals it makes lactic acid, and in plants and yeast it makes ethanol and carbon dioxide.
Aerobic respiration needs oxygen and releases much more energy per glucose molecule than anaerobic respiration, because glucose is completely broken down to carbon dioxide and water. Anaerobic respiration needs no oxygen and only partly breaks down glucose, making lactic acid in animals or ethanol and carbon dioxide in plants and yeast. Both make ATP.
Germinating seeds can be used to show that respiration releases heat and carbon dioxide. Living seeds in a vacuum flask make the temperature rise, and they turn hydrogen-carbonate indicator from red/orange to yellow or limewater cloudy. Boiled, dead seeds are the control: they show no change, proving the effects are caused by living seeds respiring.
Exam questions
6 exam-style questions (19 marks), each with its mark scheme.
Answer the questionsFlashcards
11 cards: flip them, mark what you knew, and practise the rest.
Practise the cardsMind map
The whole of structure and functions in living organisms on one page, so you can see where this subtopic fits.
Open the mind mapPractical sheet
Method, variables, risks and a results table, with questions to answer.
Print it
Free PDFs to print or save.
Quick check questions
What is the name of the molecule that provides energy for cells?
Show answer
ATP (adenosine triphosphate).
State the word equation for aerobic respiration.
Show answer
glucose + oxygen → carbon dioxide + water.
What is made in anaerobic respiration in animals?
Show answer
Lactic acid.
What is made in anaerobic respiration in plants?
Show answer
Ethanol and carbon dioxide.
Why are boiled seeds used as a control in the respiring seeds practical?
Show answer
They are dead, so they show that any changes are caused by living seeds respiring.
Frequently asked questions
How is respiration different from breathing?
Respiration is a chemical reaction in every living cell that releases energy from glucose to make ATP, while breathing is the movement of air into and out of the lungs. Breathing, or ventilation, supplies the oxygen needed for aerobic respiration and removes the carbon dioxide it produces, but the two are separate processes.
What is ATP used for in cells?
ATP provides energy for cells. When ATP is broken down to ADP and phosphate, energy is released for muscle contraction, active transport, making large molecules such as proteins from amino acids, cell division and growth, and sending nerve impulses. The ADP and phosphate are then reused in respiration to make more ATP.
What is anaerobic respiration in yeast?
Anaerobic respiration in yeast is the breakdown of glucose without oxygen to make ethanol and carbon dioxide: glucose → ethanol + carbon dioxide. Plants respire anaerobically in the same way. Because glucose is only partly broken down, much less energy is released per glucose molecule than in aerobic respiration.
Why does aerobic respiration release more energy than anaerobic?
Aerobic respiration releases more energy because glucose is completely broken down to carbon dioxide and water using oxygen. In anaerobic respiration glucose is only partly broken down, to lactic acid in animals or to ethanol and carbon dioxide in plants and yeast, so much less energy is released per glucose molecule to make ATP.
How do you show that germinating seeds respire?
Germinating seeds can be shown to respire because they release heat and carbon dioxide. Living seeds in a vacuum flask make the temperature rise, and they turn hydrogen-carbonate indicator yellow or limewater cloudy. Boiled seeds are dead, so they cannot respire; as the control, they show the changes are caused by living seeds, not the apparatus.
Written and checked against the Edexcel IGCSE Science Double Award (4SD0) specification · Updated October 2026